This disclosure relates to methods of reducing color in alkanolamine compositions and the reduced-color compositions produced thereby.
Alkanolamines such as triethanolamine are used primarily as emulsifiers and surfactants in a wide variety of industrial and consumer product applications. Such applications include household goods, for example dishwashing liquids, detergents, cleaners, polishes, cement production, cosmetics such as personal care products including shaving creams, medicine, printing, metalworking fluids, paints, lubricants, and electroless plating, to name a few. A key property to assess the quality (and thus value) of pure alkanolamines such as triethanolamine is color. In general, the less color, the higher its value. Thus, alkanolamine producers commonly have two objectives in providing pure alkanolamines: inhibiting color formation, and reducing color in discolored alkanolamines. Both of these objectives are referred to herein as “reducing color.”
Fractional distillation of crude alkanolamines such as triethanolamine can readily produce a pure product that is initially colorless or near colorless. Such initially colorless product, however, can gradually develop color during storage, even upon storage in sealed containers in the dark. This color development can include initial pinking, followed by yellowing, cumulative yellowing, and further darkening to the extent of eventual formation of a brown color. Discoloration is even more rapid if the alkanolamine is exposed to light. The phenomenon of alkanolamines such as triethanolamine and other ethanolamines turning color is described, for example, in “SRI International, Process Economics Program Report no. 193” of January 1991, pp. 6-9 and 6-10.
Various methods to reduce color in triethanolamine have generally not been entirely satisfactory for various reasons. For example, discolored triethanolamine can be treated with additives such as potassium or sodium borohydride, ethylenediamine or its di-, tri- and tetra-homologs, alkali or alkaline earth metal borates, alkanolamine esters of boric acid, hydrazine, alkylene oxides, hydroxylamine, sodium hydroxide, sodium sulfites, or sulfurous acid. Hydrogen has been used to reduce the color of triethanolamine in the presence of heterogeneous hydrogenation catalysts at temperatures above 90° C.; phosphorous acid, hypophosphorous acid, phosphines, polymeric solid acid catalysts, activated clay, porcelain, silica gel or alumina gel has been added to triethanolamine before or during vacuum distillation of triethanolamine; and passing triethanolamine through a fixed bed of activated carbon has been used to improve its color. A number of these methods suffer from the drawback that they do not sufficiently remove color from discolored triethanolamine to the extent desired to give a colorless or near colorless product. Other methods can reduce color but cannot maintain a color-free product after color removal for a desired period of time. Moreover, certain methods to reduce color in triethanolamine have recently raised environmental and health concerns.
Other prior art techniques for reducing color in alkanolamines such as triethanolamine require two-stage vacuum distillation as described in US2004158102); use of a rectifying device as described in CN201524433U; use of a two-serial mixing and displacement apparatus as described in RU2430085C1; using a device for continuous manufacture and separation of triethanolamine (US2004127748A1), use of equipment made from substantially nickel-free alloy steel as described in U.S. Pat. No. 4,567,303; use of dividing wall columns as described in US20140061020A1 and CA2540246A1; use of a device comprising a distillation column and a downstream column as described in U.S. Pat. No. 8,466,323; as well as use of electromagnetic radiation, third column distillation, and short path and thin film evaporators. Disadvantages associated with these techniques include high investment cost for special equipment, cost for disposal of spent reagent after its use, as well as handling of hazardous materials.
There accordingly remains a need in the art for methods reducing color in alkanolamines such as triethanolamine, for maintaining reduced color in alkanolamines, as well as such reduced-color compositions.